The aim of this application for the second-year-period of the renewal (extension) of our grant R01 CA 21088, is to enable us to complete the long-term, experiments on transplacental two-stage carcinogenesis, to continue perfecting our newly-designed in vivo (transplacental) test for promoting action, and to undertake the testing of a number of substances suspected of acting as tumor promoters in man's environment. Since the testing method has already proven itself, in our hands, to provide reliable results, the attempted improvements in the technique involved refer mainly to changes in dosage and timing (during the gestation period) of the 3H-thymidine, as initiator.